Gauge your ability with past exams. Here you go…
http://biolympiads.com/past-papers
https://www.acs.org/content/acs/en/education/students/highschool/olympiad/pastexams.html
https://www.aapt.org/physicsteam/2016/exams.cfm
https://artofproblemsolving.com/wiki/index.php?title=USAMO_Problems_and_Solutions